1. Field of the Invention
The present inventi on relates to a chemical mechanical polishing (CMP) method having applicability to the manufacture of ferroelectric random acc ess memory capitors.
2. Description of the Related Art
There is currently a major effort in semiconductor companies, world-wide, to commercialize high dielectric constant and ferroelectric thin films in advanced DRAMs and ferroelectric random access memories (FeRAMs), respectively. These materials include BaSrTiO.sub.3 (hST) for DRAMs and PbZrTiO.sub.3 (PZT) and SrBi.sub.2 Ti.sub.2 O.sub.9 (SBT) for FeRAMs.
It is well known that these materials require electrodes made from noble metals or noble metal alloys such as Pt, Ir, IrO.sub.2, Pt-Ru, etc., and sub-micron patterning of both the noble metals and the ferroelectric films is very difficult because of the absence of volatile products for the elemental constituents. State-of-the-art dry etching processes for Pt and Ir are known to have fundamental difficulties due to the predominantly physical (not chemical) mechanism for material removal, resulting in formation of unwanted structures (sometimes called "ears") at the edges of the electrodes.
Besides the difficulties in patterning sub-micron capacitors of this type, for high memory density it is also important to fabricate the capacitors directly over a conductive plug to contact transistors, in order to reduce the area of the memory cell. This geometry (capacitor over plug) is also known as a stack capacitor configuration. For the conventionally employed materials, e.g., polysilicon or tungsten for the plug, a barrier layer is needed to prevent oxidation of the plug and diffusion of the plug material (p-Si or W) through the noble metal bottom electrode. To overcome such problems, it is desirable to use geometric means to protect the plug/barrier/electrode interfaces from exposure to oxidation.
An alternative to the stack capacitor is a trench capacitor, which utilizes an enhanced surface area capacitor on the walls of a trench that is etched directly into doped silicon. In such trench capacitors, the bottom electrode contact is not to a conductive plug (p-Si or W), but to the conductive substrate itself, and the requirements for the baprier are similar to the stack onfiguration. In trench capacitor architecture, the memory cell's transistors are formed on the surface of wafer adjacent to the top of the trench capacitor.
Ferroelectric capacitors planarized using chemical mechanical polishing are also more versatile for monolithic integration of ferroelectric memory or dynamic random access memory (DRAM) with logic IC ("embedded memory"), since the 4-6 levels of metal needed for logic IC's places additional demands on the planarity of the underlying structures, compounded by the need for surface flatness in high resolution microlithography, in ord er to stay within the aligner's specified depth of focus.
An additional constraint is economic. It is essential to minimize the number of processing steps as much as possible, and to achieve the highest possible yield for IC devices that are manufactured.
By way of background to the ensuing description of the present invention, a description is set out below of machines used in conventional CMP process operations, polishing pads and slurry compositions used in chemical mechanical processing.
Three types of mechanical, rotating actions are typically employed in conventional CMP machines. One such type has a rotating wafer carrier. Another revolves the abrasive pad. A third type spins both the wafer carrier and the abrasive pad simultaneously. The polishing pad is presoaked and continually rewet with a slurry consisting of various particles suspended in a solution. The polishing particles range in size from 30 to 1100 nanometers. The suspension solution generally comprises a diluted base or acid for polishing oxide and metals respectively. Upon completion of the planarization, the wafers go through a post-CMP clean process in which the residual slurry, ground oxide/metal particles, and other potential contaminants are removed. Most IC manufacturers use a combination of deionized (DI) water rinses and nitrogen air drying to accomplish the post-CMP decontamination.
The two most common uses of CMP are oxide and metal (tungsten) plug planarization. The two most essential components of the CMP process are the slurry and polishing pad.
The polishing pad, generally a polyurethane-based material, performs two primary functions. The polymeric foam cell walls of the pad aid in removal of reaction products at the wafer surface and the pores within the pad assist in supplying slurry to the pad/wafer interface.
Progressively more research efforts have focused on further understanding empirical results of the intimate contact between the pad and the pre-planarization surface. Several "peculiarities" were originally noticed in the material removal rate (RR) as a function of time, pressure, and velocity. Many CMP users noticed that the RR decreased tremendously as high throughput processes were attempted. Research showed that deformation of the pad resulted from the brittle, hard surface of the IC surfaces. The asperity of the pads, or surface roughness due to the type of pad material, the density of "pockets", and abrasive characteristics, was found to decline due to pad layer removal. To alleviate this problem, called "glazing", the pad was conditioned after an experimentally determined amount of time, or wafer runs. The conditioner was composed of a base material (metal), a diamond grit (for cutting), and a grit-bonding material (Ni plating). The plating bonded the diamond grit to the base material. The conditioner then effectively removed the top layer of the pad until excess, non-desired particles were removed and the nominal surface characteristics of the pad were present. Although this approach allowed the same pads to be used for an extended amount of time, it also resulted in other complications.
Specifically, the physical bonding of the diamonds and base material ruptured relatively easily during the conditioning process. New methods of bonding were pursued as well as enhanced post-conditioning cleaning. When the diamonds were chemically bonded to the base material, the additional strength made grit detachment less prominent. The new bonding method allowed a factor of ten more wafers to be polished with the same conditioning pads as compared to the number that were able to be polished with the physically grit-bonded pads.
Pad conditioning plays a larger role in planarization of oxide than in planarization of metals since metals tend to have a higher degree of hardness in relation to the pad material.
To aid in transporting slurry to the pad/wafer interface, new pad structures have been designed. Karaki-Doy and other developers have designed polishing pads with continuous grooves in concentric ellipses across the entire pad. This structure was found to deliver the slurry more uniformly to the interface and to augmented the amount of debris removal resulting from the CMP process. Most conventional pads consist of "pockets" within the polyurethane which are prone to clogging due to residual debris discharged during the process. Consequently, Karaki-Doy and other developers placed the grooves in the surface of the pad, and noticed an increased longevity in the conditioning-to-conditioning life of their pads over common types of pads.
Another key element in the amenabilty of the pad to planarize uniformly is the IC device density and relative layer heights (critical dimensions, CD) of the structure undergoing CMP. CMP tends to polish smaller, individual features faster than larger, more densely packed features. The oxide removal rate over features 15 mm in width is 60-80% greater than the oxide removal rate over features four times that width. Denser or larger features tend to distribute applied load pressure over a larger area than smaller features. Since the removal rate and pressure are directly related in the direct contact mode, the removal rate decreases since the effective, local pressure decreases. The same principles apply when adjacent layers have a larger height contrast. "Taller" features will be planarized quicker, depending on other dimensions and the proximity to other devices.
The foregoing factors add complexity to IC design. If IC manufacturing processes require CMP procedures, device dimensions and density are critical and require close scrutiny.
The slurry composition used in CMP comprises abrasive particles suspended in a solvent, or solution. Key factors in the effectiveness of the slurry include particle size and shape, solution pH, weight percent solids of the slurry, the quantity of the slurry delivered, and the reaction time involved.
The purpose of the slurry is simple, yet understanding and modeling all the mechanical and chemical reactions involved is extremely difficult. Essentially, the surface of the material being polished is chemically altered to a softer composition which is then mechanically removed by the pad and slurry abrasives. Thus, the slurry provides both chemical and mechanical forces in the CMP process. Oxide slurries are usually KOH-- or NH.sub.4 OH-based with a fumed silica abrasive and a high pH. Metal slurries are relatively new and largely experimental, yet the most common are ferric nitrate with an alumina abrasive and low pH. Some polysilicon and polyimide slurries exist, but are still in the prototype and developmental stages.
To date, most of the research devoted to development of slurry compositions has focused on oxide slurries instead of metal slurries. Due the numerous desirable characteristics of tungsten plugs, more attention is being directed to development of selective metal slurries. IBM has developed metal slurries with a tungsten:oxide selectivity of 120:1. This type of planarization is essential to the fabrication of multilevel metals and interlevel connects. Ideally the slurries investigated will produce high removal rates, high selectivity, local uniformity, and good planarity. Since "perfect" slurries do not currently exist, inevitable trade-offs have made in evolving acceptable commercial slurry formulations.
Due to the chemical nature of CMP, various studies have evaluated the influence of differing amounts of slurry introduced at the wafer/pad interface. In the case of oxide slurries, it is believed that the water in the solution reacts with the silicon oxide in the reaction as follows EQU (--Si--O--Si--)+H.sub.2 O=&gt;2(--Si--OH)
This equation shows the base portion of the entire, repetitive chemical structure (. . . --Si--O--Si--Si--O--Si-- . . . , OH--Si-- . . . --Si--OH) for simplicity. The reaction at the interface primarily occurs between molecules on the surface of the wafer and the silica particles in the slurry, since water has a low diffusivity in silicon oxide. Increasing the temperature directly increases the removal rate since the diffusivity of the water rises (specifically the diffusion constant of water in oxide). The most effective pH levels for oxide planarization lie between 9.7 and 11.4.
In the case of metal slurries, the composition is even more critical. Typical slurries incorporate an oxidizer or naturally dissolved oxygen additives to adjust pH levels, and either alumina or colloidal silica abrasives. The oxidizer changes the oxidation state of the metal and consequently produces metal ions. The top oxidized metal layer is more brittle and easily removed with the embedded abrasive particles. If the oxidation potential is too aggressive or the resulting metal compound too soluble, metal corrosion can occur as a result of wet etching. Alloys, galvanic actions, and precise oxidation states (oxidizers) are employed to slow down wet etching and limit the metal corrosion.
Two other key issues relating to the choice of slurries deal with post-CMP clean-up and the introduction of mobile ions to the wafer. Depending on the chemical reaction, oxide slurries can introduce various contaminants to the wafers surface. In terms of particle sizes, KOH-based slurries introduce a larger quantity of 2000 Angstrom particles than do the NH.sub.4 OH slurries. That difference translates into a higher probability of scratches (e.g., up to 7 times greater, according to some studies) on the wafer surface when using KOH slurries. NH.sub.4 OH slurries also produce a lower concentration of mobile ions than KOH-base slurries, and leave residual films that are easier to remove than the residue from KOH slurries. Environmentally, however, KOH-base slurries afford advantages over NH.sub.4 OH slurries. No ammonia smell exists when using KOH slurries, KOH slurries are less prone to settle in cleaning tanks and CMP machines, and KOH slurries are more stabile in terms of pH, and less temperature dependent than NH.sub.4 OH slurries.
Although CMP has revolutionized global planarization technology, some significant problems exist. One of the major difficulties is in-situ measuring of the amount of material removed form the wafer's surface. Due to inaccurate models, many results of CMP machines are difficult to reproduce and the machines themselves do not exhibit the ability for precise process control. This also leads to difficulty in analyzing feedback, or using in-situ measurements, to make adequate and appropriate process alterations to alleviate process complications. Some CMP slurry analyzers have been designed to measure and detect particle sizes in order to ascertain the abrasive characteristics of slurries more accurately. A few endpoint detection devices, like a stylus profiler, have been developed to monitor removal rates as well. Such efforts will aid in more precisely controlling the entire CMP process, but the analysis techniques and instruments have not been developed to a state of high commercial precision.
Thus, commercial CMP is the focus of substantial development effort, but in essence it continues to comprise the simple unit operations of:
1. reaction of an exposed layer of material (e.g., an insulating inorganic metal oxide and/or noble metal) to produce a wafer-adhered material whose hardness is less than the hardness of the abrasive and whose adhesion to the substrate is less than the original pre-reaction layer; and PA1 2. removal from the substrate of the aforementioned reaction product material using a polishing slurry (abrasive medium).
Illustrative CMP slurry compositions (by principal reaction type) for insulating inorganic metal oxides include the compositions set out below:
A. Acidic or basic aqueous solution:
______________________________________ HCl, H.sub.2 SO.sub.4 0.01M or greater KOH, NaOH, NH.sub.4 OH 0.01M or greater ______________________________________
The art has directed improvements to alkali-containing solutions via aqueous or alcohol solutions of fluorinated silicon oxide particles, specifically the use of H.sub.2 SiF.sub.6.
The slurry composition comprises Al.sub.2 O.sub.3 and/or SiO.sub.2 aqueous solution.
B. Oxidizing agent (with reduction potential, Eo, greater than IV):
______________________________________ H.sub.2 O.sub.2 35 vol. % or greater nitrates, chromates, permagnates, O.sub.3 and F.sub.2 ______________________________________
The slurry composition comprises Al.sub.2 O.sub.3 and/or SiO.sub.2 aqueous solution.
C. Halogenated or psuedohalogenated material (in inert atmosphere):
______________________________________ POCl.sub.3, SOCl.sub.2 100% or combined with a dry solvent (solvents: toluene, ethers, ketones, DMSO, etc.) P(SCN).sub.3, (SCN).sub.2, 20-100 vol. % S(SCN).sub.2, Hg(NCS).sub.2, Hg(NCO).sub.2, AgNCO, CH.sub.3 NCO, C.sub.6 H.sub.5 NCO, BrCN ______________________________________
The slurry comprises a non-aqueous halogenated or pseudohalogenated reactant, and preferably includes a liquid organic ligand precursor (e.g., cyclic, acyclic, polycyclic, or aromatic compounds) which upon reaction with the halogenated or pseudohalogenated material form a metal-organic coordination complex which is heterocyclic.
The occurrence of dishing or polishing flaws, particularly with the CMP of soft metals such as Al, Cu or Ag, have been mediated in part by storage/delivery of a polishing agent slurry at reduced temperatures where flocculation or precipitation of the slurry is minimized. In addition, agitation of the storage tank for the polishing media (to inhibit flocculation), as well as temperature and velocity control of the polishing wheel/surface have been shown to improve CMP homogeneity.
The CMP pad wears at an exponential rate during its initial use and then wears linearly with time. Further, the CMP pad does not remove material uniformly as the pad continues in use. These factors make it difficult to maintain an acceptable removal rate and uniformity in the CMP operation.
Individual spatial dimensions of the top electrode/ferroelectric material/bottom electrode (TE/FE/BE) capacitor in a typical FeRAM are on the order of 100 nm. In order to minimize damage/inhomogeneity during the CMP of this layered structure, the maximum abrasive particle size should be much less than the minimum feature size in the device.
The art continues to seek improvements in the CMP process and in the fabrication of ferroelectric devices such as FeRAMs.